Integrated circuits typically include a number of input/output pins which are used for communication with additional circuitry. For example, an integrated memory device such as a dynamic random access memory (DRAM) includes both control inputs for receiving memory operation control signals, and data pins for bi-directional data communication with an external system or processor.
The data transmission rate of modern integrated circuits is primarily limited by internal circuitry operating speeds. Communication networks can typically transmit signals between circuitry at a rate that is faster than the capacity of some integrated circuits. To address the need for faster circuits, a group of integrated circuits can be combined on a common bus. In this configuration, each integrated circuit operates in a coordinated manner with the other integrated circuits to share data that is transmitted at a high speed. For example, a group of memory devices, such as DRAMs, static RAMs, or read only memories (ROM), can be connected to a common data bus. The data rate of the bus may be substantially faster than the feasible operating speed of the individual memories. Each memory, therefore, is operated so that while one memory processes received data, another memory receives new data. By providing an appropriate number of memory devices and an efficient control system, very high speed data transmissions can be achieved.
As the transmission rate of the data communication signals continues to increase, new circuitry and methods are needed to accurately transmit data from each integrated circuit. One proposed solution is a bus driver described in U.S. Pat. No. 5,254,883. This bus driver circuit uses parallel output transistors that single-endedly couple the bus to a power supply. The output transistors are fabricated in different sizes and selectively activated to control the bus current. This approach is directed towards accurate bus current control to establish the bus voltage at an accurate logic voltage level. However, the approach lacks control over the transition of the bus voltage between logic voltage levels.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a high speed output driver circuit which is fully adjustable, and offers control of the transition of the bus voltage between logic voltage levels.